1. Field of the Invention
The present invention relates to a method of manufacturing plastic optical elements that are to be used in optical apparatuses such as a digital camera and a copying machine.
2. Description of the Related Art
There has been a growing demand for manufacturing of a thick optical element by injection molding. However, as the thickness of an optical element increases, a stress increases due to a difference between the degree of shrinkage on curing of a plastic surface layer that hardens first and the degree of shrinkage on curing of a plastic inner portion that hardens subsequently while the optical element is being molded. Thus, there have been problems of vacuum bubbles (voids) generated within the optical element and a residual internal stress within the optical element. In addition, since the time required to cool such a thick optical element in a metal mold becomes very long as the thickness of the optical element increases, there has been a problem in that the length of a molding cycle significantly increases. As a measure to address these problems, Japanese Patent Laid-Open No. 63-315216 discloses a method of manufacturing a plastic optical element, the method including arranging a premolded plastic member in a metal mold and covering the member with a molten plastic so that the member becomes integrated with the molten plastic by simultaneously injecting the molten plastic onto front and rear surfaces of the member.
In Japanese Patent Laid-Open No. 63-315216, in order to prevent the member from being displaced by a resin pressure exerted by a covering resin, member holding portions are formed on the outer peripheral surface of the member, and the member holding portions are brought into contact with the metal mold so as to directly hold the member. However, in this method, a bending stress is generated in the member holding portions due to a difference between internal pressures of molded resins simultaneously injected onto the front and rear surfaces of the member. There have been problems in that the stress causes deformation of the member, and the birefringence of the member increases. As a result, the optical performance of a thick lens formed of the member covered with the resin is reduced, and in some cases, cracks occur in the member holding portions.